1. Field of the Invention
The present invention relates to an apparatus for use in supplying auxiliary pressurized hydraulic fluid to a subsea equipment assembly near the ocean floor.
2. Description of the Prior Art
As offshore drilling operations progress into deeper waters, the use of fixed, bottom-supported drilling structures decreases. Instead, dynamically-positioned drilling vessels are used in water depths usually greater than 1,000 feet. To drill a deepwater well, one of these drilling vessels is usually positioned above a subsea wellhead located upon the ocean floor and a pipe assembly, commonly called a riser, is extended downwardly from the vessel to the subsea wellhead.
The lower end of the riser usually contains hydraulically actuated well control valves and equipment coupling devices used to connect the lower end of the riser to the subsea wellhead. Pressurized hydraulic fluid supplied from the drilling vessel via hydraulic cables is used to actuate these hydraulic devices.
At times, the hydraulic cables may be damaged during drilling operations, severing the source of pressurized hydraulic fluid that is supplied to the hydraulically-actuated devices.
Without the assistance of divers or remotely operated vehicles (R.O.V.s), the entire riser assembly would have to be retrieved to the surface in order to repair the hydraulic system failure, an operation that would require the retrieval of thousands of feet of pipe riser at a cost of up to a million dollars.
Since divers cannot be safely used to repair subsea equipment which is located at depths which may approach 7,000 feet, the task of repairing the hydraulic system failure would fall to an R.O.V., if the R.O.V. is capable of performing the hydraulic system repair operation. As set forth in U.S. Pat. No. 3,463,226, entitled "Underwater Wellhead Apparatus", issued Aug. 26, 1969, to Glenn D. Johnson, and in U.S. Pat. No. 3,354,658, entitled "Apparatus for Performing Underwater Operations", issued Nov. 28, 1967, to Sam Leonardi, and R.O.V. may supply air to a sunken vessel or turn bolts attached to a subsea wellhead by a manipulator arm carried by the R.O.V.
However, it may not be possible for the R.O.V. to hydraulically connect an auxiliary source of pressurized hydraulic fluid to the subsea blowout preventer stack, lower riser package or xmas tree to repair or bypass the damaged hydraulic system. Even during the most optimistic R.O.V. operating conditions, the probability of successfully connecting two small diameter high pressure hydraulic cables is minimal at best. The chances of completion of a successful hydraulic connection between the R.O.V. and the riser, blowout preventer stack or xmas tree are decreased even further if the riser, while suspended beneath the drilling vessel, moves upward and downward due to ocean waves which impact the drilling vessel. The operator of the R.O.V., or even a diver if the water depth is shallow enough, would be presented with a "moving target" as the riser vertically oscillates through the water.
Accordingly, it is essential to provide a method and apparatus to allow the hydraulic connection of a source of pressurized hydraulic fluid to a riser or subsea blowout preventer stack or xmas tree, under all possible subsea conditions, without having to retrieve the entire riser assembly to the surface to complete the hydraulic connection.